My eldest son was bullied when he first started school and I couldn't work out what was happening and it took me too long to learn that you have to deal with it and deal with it very forcefully straight away.
I dithered at first because I couldn't understand what was happening and thought my son was overreacting to some mild ribbing (he found it very difficult to explain to me exactly what was said and done, which made it confusing for me). Then once I caught the bullies in the act and realised that he was being horribly bullied by much older boys who were clever enough not to get spotted by the teachers and dinner ladies. I've never forgiven myself for not dealing with his obvious distress in a strong enough way.
You need to talk with BO and tell her that your dd came home from camp in distress and that if she is ever to go back to brownies, you need to know that BO will be watching like a hawk and that dd knows that she can go straight to BO if there is a problem and trust her to sort it out.
Does dd want to go back to Brownies?
Of course it is possible that she is making this up because she has suffered at the hands of bullies in the past - I very much doubt that she has totally invented all the bullying. Even if this is so, you need to work o helping her feel more secure, not make her feel bad about it.
I would be straight with your dd about the fact that you are confused that her bf doesn't seem to know about stuff. I would explain that you really need her to tell you exactly what happened. Maybe at some point, you could also tell the story of "The Boy Who Cried Wolf" to emphasise how important it is that she can tell you the truth.
If you make it clear that you will help her and not be cross if she did exaggerate a bit she may well be able to be straight with you. If what happened on camp doesn't sound too bad to you, you can talk through with her the fact that the other girls probably didn't realise how hurtful they were being, but don't undermine her right to be upset if she was upset by it, otherwise you will undermine her self esteem further - she could feel that not only is she worthy of teasing/bullying, but she is stupid to be upset about it.
There all all sorts of explanations for the bf saying nothing "bad" happened.
The one thing you do know is your dd is distressed and you just need to focus on helping her to feel supported.
My twenty pence worth - I do feel very strongly about this because my son spent years regaining trust in school and trust in me to protect him and I just wish I hadn't stuffed it up in the first place.